Dynamic Effects in the Reverse Flow Velocity Field

At high edgewise flow speeds, reversed flow causes excursions in pitching moment and blade torsion on the rotor blades of helicopters and wind turbines. Our prior work has shown that a yawed fixed wing at angle of attack in reversed flow generates a sharp-edge vortex. The sharp-edge vortex is a primary feature of the flow under the rotor blade. During operation as a rotor blade at high advance ratio in a wind tunnel, stereo PIV results show that the well-formed sharp-edge vortex at 240 degrees azimuth resembles that on a forward-swept wing. This vortex stops growing before 270 degrees and convects with the blade by 300 degrees. Static pressure computed from interpolated velocity data show the effects of vortex-induced radial pressure gradient. This explains the finding of inboard-directed radial flow at 300 degrees azimuth, overcoming centrifugal effects. Viscous stress is shown to have only a minor effect on the static pressure field computation.